Integrated circuits are made possible by processes which produce intricately patterned material layers on substrate surfaces. Producing patterned material on a substrate requires controlled methods for removal of exposed material. Chemical etching is used for a variety of purposes including transferring a pattern in photoresist into underlying layers, thinning layers or thinning lateral dimensions of features already present on the surface. Often it is desirable to have an etch process which etches one material faster than another helping e.g. a pattern transfer process proceed. Such an etch process is said to be selective to the first material. As a result of the diversity of materials, circuits and processes, etch processes have been developed with a selectivity towards a variety of materials.
A wet HF etch preferentially removes silicon oxide over other dielectrics and semiconductors. However, wet processes are unable to penetrate some constrained trenches and sometimes deform the remaining material. Dry etches produced in local plasmas (plasmas within the substrate processing region) can penetrate more constrained trenches and exhibit less deformation of delicate remaining structures. However, local plasmas can damage the substrate through the production of electric arcs as they discharge.
A Siconi™ etch is a remote plasma assisted dry etch process which involves the simultaneous exposure of a substrate to H2, NF3 and NH3 plasma by-products. Remote plasma excitation of the hydrogen and fluorine species allows plasma-damage-free substrate processing. The Siconi™ etch is largely conformal and selective towards silicon oxide layers but does not readily etch silicon regardless of whether the silicon is amorphous, crystalline or polycrystalline. Silicon nitride is typically etched at a rate between silicon and silicon oxide, but the selectivity of silicon oxide over silicon nitride is typically not as pronounced as the selectivity of silicon oxide over silicon. The selectivity provides advantages for applications such as shallow trench isolation (STI) and inter-layer dielectric (ILD) recess formation. The Siconi™ process produces solid by-products which grow on the surface of the substrate as substrate material is removed. The solid by-products are subsequently removed via sublimation when the temperature of the substrate is raised. As a consequence of the production of solid by-products, Siconi™ etch process can deform delicate remaining structures as well.
Methods are needed to selectively remove silicon oxide while not forming solid by-products on the surface since their formation may disturb delicate structures on a patterned substrate.